When I change the individual or total group volume in the iOS Sonos app, I can see the sliders for each speaker jump around and I can hear various speakers leave and re-enter the group. Sometimes it settles on the volume I set. Often it doesn’t.
Also, more than once I’ve had to stitch off and switch on speakers so that they can be found by Sonos.
Sounds like your devices are weak on wifi connection.
I suggest you send a diagnostic after that happened and call Sonos support. Tell them the diag confirmation number and they can take a look at the diag information.
Check SNR from the app. Gear - device under Your System - Network. Best to have a SNR of 45 dB or more.
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